Flow Sensor Calibrated by Volume Changes

ABSTRACT

An infusion pump and method are provided which combine flow rate measurements calibrated by accurate volume measurements over time.

BACKGROUND

The present disclosure relates to fluid flow control systems, such as intravenous infusion pumps, and more particularly to feedback control infusion pumps with flow and volume sensing.

In some situations, such as with intravenous infusions, the flow rate range requirement is very broad, covering more than four orders of magnitude of rate, e.g., from about 0.1 ml per hour to about 6,000 ml per hour. If any sort of low flow rate sensitivity is to be expected, then the response of a flow sensor should be logarithmic or at least non-linear. The absolute calibration of such a flow sensor would be very challenging, especially at low flow rates. Even without the requirement for calibration, it is difficult to find a commercially available flow sensor that services this flow rate range.

Certain infusions have historically been managed by air pressure delivery systems, most commonly found in the operating room and in emergency situations. Prior art attempts have been made to determine the flow rate via pressure monitoring and control. For example, U.S. Pat. No. 5,207,645 to Ross et al. discloses pressurizing an IV bag and monitoring pressure to infer flow rates. However, the prior art systems lack independent flow sensing and, therefore, do not offer enough information to provide accurate and safe infusions. Under the best of circumstances, there is not enough information in the pressure signal alone to provide the accuracy needed for intravenous infusion therapy.

Furthermore, there are a number of likely failure modes that would go undetected using this pressure signal alone. An infusion pump must be able to respond to events in a relevant time frame. International standards suggest that a maximum period of 20 seconds can lapse before fluid delivery is considered “non-continuous.” As an example, for an infusion of 10 ml per hour, the system would want to resolve 20 seconds of flow, which corresponds to 0.056 mL. This volume represents one part in 180,000 of the total air volume. Temperature-induced change in pressure brought about by a normal air conditioning cycle is far greater than this signal. The measurement of pressure alone is not adequate for an intravenous infusion device. No general-purpose full-range, infusion devices using pressure-controlled delivery are known to be on the market.

It would therefore be useful to develop a fluid delivery device and a control system and method therefor that could control the rate fluid flow based on a high resolution, highly responsive fluid flow sensor that could be calibrated in situ by correlating its signal with changes in fluid volume over time.

SUMMARY OF THE DISCLOSURE

In one aspect, a fluid control system that combines flow rate and absolute volume calculations is provided. In further aspects, a fluid delivery device and methods employing the same are provided.

An advantage of the present disclosure resides in its accuracy. The present disclosure offers two novel views of flow control, one operating over many minutes, and the other operating many times per second, which is analogous to having both a speedometer and an odometer, so that a steady speed may be maintained, while still making adjustments to arrive at one's destination on time. The high degree of accuracy is advantageous for infusion of vasoactive drugs, since the flow should be as continuous as possible and the flow rate resolution should be high enough to resolve a 1% change in requested flow rate. Likewise, for long term doses (e.g., 12-72 hours), it is important to complete the dose on time. Also, for longer-term infusions, the accuracy of the system should not degrade over time. There are clinical and operational problems with infusions that are completed too soon (e.g., increased chance of clotting off the IV line) or too late (e.g., unused medication, biohazard, etc.). For intermittent infusions, the time the dose starts is important, so that the peak serum levels of the intermittent medication occur as ordered. For market acceptance, the device should perform well according to worldwide testing standards, including the integrated flow volume “trumpet curve.”

An additional advantage of the present disclosure resides in its simplicity. Instead of a complex electromechanical assembly of gears, levers, motors, and switches, a sensor-based flow regulation system can be relatively simple and low cost in its construction.

In operation, a bag of fluid to be infused is set inside a rigid container. The device automatically measures the volume of liquid in the bag. A user input device is provided to allow the user to input (1) how fast the fluid should flow; (2) when the infusion should be complete; and/or (3) the amount of time the infusions should take to finish.

In certain embodiments, the present disclosure is capable of operating over an extended range of flow rates, e.g., from below 1 milliliter per hour to 6,000 milliliters per hour. At all flow rates, the fluid flows continuously without the characteristic flow disruptions of a typical motor driven large-volume pump.

Other benefits and advantages of the present disclosure will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention.

FIGS. 1 and 2 are perspective and side views, respectively, of an infusion pump in accordance with an exemplary embodiment.

FIG. 3 is a functional block diagram showing the fluidic connections of a volume measurement system according to an exemplary embodiment.

FIG. 4 is a functional block diagram showing the control elements of a volume measurement system according to an exemplary embodiment.

FIG. 5 is a functional block diagram showing the sensing elements of the system.

FIG. 6 is a flow chart of an exemplary method for calculating the volume of liquid to be infused.

FIG. 7 is a perspective view of the exterior of the flow sensor

FIG. 8 is an exploded view of the flow sensor.

FIG. 9 is a cross-sectional view of the flow sensor

FIG. 10 is a perspective view of the flow sensor housing.

FIG. 11 is a graphical representation of force balancing in the flow sensor.

FIG. 12 is a flow chart outlining an exemplary method of calculating flow rate based on pressure decay.

FIG. 13 is a block diagram illustrating an exemplary system having plural, independent methods for measuring flow rate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, wherein like numerals reference numerals are used to indicate like or analogous components throughout the several views, FIGS. 1 and 2 depict an exemplary volume and flow measurement system in accordance with an exemplary embodiment of the present invention. The system includes a pressure frame 10 that is of known total volume and contains within it an air bladder 20 and a flexible bag 30 that contains within it a liquid 40 to be infused.

Referring now to FIG. 3, the air bladder 20 is connected to an air pump 50 via a bladder connection line 608, a bladder valve 106, and a bladder valve line 606. The air bladder 20 may be vented to atmosphere via a bladder vent valve 108.

A calibration tank 60 of known volume is connected to the air pump 50 via a tank connection line 604, a tank valve 102, and a tank valve line 602. The tank 60 may be vented to atmosphere via a tank vent valve 104.

The liquid 40 is fluidically coupled to an output 500 via a liquid drain line 610, going through a fluid flow resistor 400 and through an output line 612. The liquid 40 may be, for example, a medication fluid, intravenous solution, or the like, and the output 500 may be, for example, a patient or subject in need thereof. An inline flow sensor 900 is provided in the line 612, as described in greater detail below.

The tank 60 is connected to a tank pressure sensor 204 and a tank temperature sensor 304. The bladder 20 is connected to a bladder pressure sensor 202 and a bladder temperature sensor 302.

Referring now to FIG. 4, an electronic module includes a processing unit 700 such as a microprocessor, microcontroller, controller, embedded controller, or the like, and is preferably a low cost, high performance processor designed for consumer applications such as MP3 players, cell phones, and so forth. More preferably, the processor 700 is a modern digital signal processor (DSP) chip that offers low cost and high performance. Such processors are advantageous in that they support the use of a 4th generation programming environment that may substantially reduce software development cost. It also provides an ideal environment for verification and validation of design. It will be recognized that the control logic of the present development may be implemented in hardware, software, firmware, or any combination thereof, and that any dedicated or programmable processing unit may be employed. Alternately, the processing unit 700 may be a finite state machine, e.g., which may be realized by a programmable logic device (PLD), field programmable gate array (FPGA), field programmable object array (FPOA), or the like. Well-known internal components for processor 700, such as power supplies, analog-to-digital converters, clock circuitry, etc, are not shown in FIG. 4 for simplicity, and would be understood by persons skilled in the art. Advantageously, the processing module 700 may employ a commercially available embedded controller, such as the BLACKFIN® family of microprocessors available from Analog Devices, Inc., of Norwood, Mass.

With continued reference to FIG. 4, the processing unit 700 controls the air pump 50 via a pump control line 750. The processor 700 controls the tank vent valve 104 via a tank vent valve control line 704. The processor 700 controls the tank valve 102 via a tank valve control line 702. The processor 700 controls the bladder vent valve 108 via a bladder vent valve control line 708. The processor 700 controls the bladder valve 106 via a bladder valve control line 706.

With reference now to FIG. 5, the processor 700 can measure pressure and temperature from the bladder 20 and tank 60. The processor 700 reads the pressure in the tank 60 via a tank pressure sensor 204, which is coupled to the via tank pressure line 724. The processor 700 reads the pressure in the bladder 20 via a bladder pressure sensor 202, which is coupled to the processor 700 via a tank pressure line 722. The processor 700 reads temperature of the gas in the tank 60 via a tank temperature sensor 304, which is coupled to the processor 700 via a tank temperature line 714. The processor 700 reads the temperature of the gas in the bladder 20 via a bladder temperature sensor 302, which is coupled to the processor 700 via a bladder temperature line 712. The processing system 700 may receive flow rate data from the inline flow sensor 900 via data line 710.

Volume Measurement

Ultimately, the objective of volume measurement is to know the quantity of liquid 40 remaining in an infusion and how that quantity changes over time.

The pressure frame 10 defines a rigid container of known volume, V_(frame). This volume is known by design and is easily verified by displacement methods. Within the pressure frame 10, there is the air bladder 20, which has a nominal capacity greater than the volume V_(frame). When expanded, the bladder must conform to the geometry of the rigid container and its contents. The volume of liquid 40 to be infused, V_(tbi), is equal to V_(frame), less the fixed and known volume of the bladder 20 itself, V_(blad), less any incompressible materials of the bag 30, V_(blag), and less the volume of gas in bladder 20, V_(gas). Once the value V_(gas) is computed, then V_(tbi) may be computed as follows:

V _(tbi) =V _(frame) −V _(blad) −V _(bag) −V _(gas)

With the following method, at any given point in time, the volume of air contained in the bladder, V_(gas), can be measured and V_(tbi) can be subsequently computed.

For purposes of economy and flexibility, the pump 50 may be an imprecise air pump, such as that of a rolling diaphragm variety, although other types of pumps are also contemplated. The output of such a pump may vary significantly with changes in back pressure, temperature, age of the device, power supply variation, etc. One advantage of the device and method disclosed herein is that they allow an imprecise pump to be used in a precision application, by calibrating the pump in situ.

FIG. 6 shows the steps leading to computation of V_(tbi). Shown as step 802, the first step is to find an optimum amount of air mass, N_(pump), to add to the bladder 20 to effect a significant pressure change, for example, on the order of about 10%. If the amount of air mass added to the bladder is too small, then the pressure change will not be measurable with accuracy. If the amount of the air mass is too great, then pressure in the bladder will increase more than necessary and energy will be wasted.

The initial pressure in the bladder 20, _(P) _(bladder1), is measured using the bladder pressure sensor 202. The tank valve 102 is set to a closed state via the tank control valve line 702 from the processor 700. The bladder valve 106 is set to an open state via the tank control valve line 706 from the processor 700. The pump 50 is activated by the processor 700 via the pump control line 750 for a period of time, S_(test), nominally, for example, about 250 milliseconds. A new measurement of the pressure in the bladder 20 is made, P_(bladder2). Based on the percent of pressure change from this pumping action, a new pump activation time, S_(pump), will be computed. This calculation needs no precision; it is only intended to find an amount of pumping that provides a significant change in pressure, P_(deltatarget), in the bladder 20, for example, on the order of about 10%.

$S_{pump} = {S_{test}*\frac{P_{deltatarget}}{\left( {P_{{bladder}\mspace{11mu} 2} - P_{{bladder}\mspace{11mu} 1}} \right)/P_{{bladder}\mspace{11mu} 1}}}$

In step 804, the pump 50 or the tank vent valve 104 are activated to increase or decrease, respectively, the pressure, P_(tank), in the tank 60, so that it approximately equals the pressure, P_(bladder), in the bladder 20. The combination of valve and pump settings required for such adjustments are shown in the table below:

Bladder Tank Pump Valve Bladder Vent Valve Tank Vent 10 106 Valve 108 102 Valve 104 Increase P_(bladder) ON OPEN CLOSED CLOSED CLOSED Decrease OFF CLOSED OPEN CLOSED CLOSED P_(bladder) Increase P_(tank) ON CLOSED CLOSED OPEN CLOSED Decrease P_(tank) OFF CLOSED CLOSED CLOSED OPEN

Adjustments made in step 804 can be made iteratively until P_(tank) is roughly equal to P_(bladder), for example, within about 5% of the relative pressure measured in P_(bladder). This does not need to be a precise process. Following the adjustment, the pressure in tank 60, P_(tank2), is recorded.

In step 806, the system is configured to increase the pressure in tank 60, as shown in the above table. The pump 50 is activated for a time period equal to S_(pump). After a delay of approximately five seconds, the pressure in the tank 60 is measured, P_(tank3). This delay is to reduce the effect of an adiabatic response from the increase in pressure in the tank 60.

In step 808, the system is configured to increase the pressure in the bladder 20, as shown in the above table. The pump 50 is activated for a period equal to S_(pump). After a delay of approximately five seconds, the pressure in the bladder 20 is measured, P_(bladder3). This delay is to reduce the effect of an adiabatic response from the increase in pressure in the bladder 20.

Because the initial pressures in the bladder 20 and the tank 60 were approximately equal, the quantity of air mass injected into the tank 60 in step 806 and into the bladder 20 in step 808 will be roughly equal, even though the pump 50 need not be a precise metering device.

We take advantage of several simplifications. First, the ambient temperature for sequential steps 806 and 808 is unchanged. Second, the atmospheric pressure during sequential steps 806 and 808 is unchanged. These conditions simplify the ideal gas law formula and allow the use of gauge pressure measurements, rather than absolute pressure.

In step 810, the volume of gas in the bladder 20, V_(gas), can be calculated with a reduced form of PV=nRT:

$V_{gas} = \frac{V_{tank}*\left( {P_{{tank}\mspace{11mu} 3} - P_{{tank}\mspace{11mu} 2}} \right)}{\left( {P_{{bladder}\mspace{11mu} 3} - P_{{bladder}\mspace{11mu} 2}} \right)}$

As examples of this calculation, if the pressure change were the same in the bladder 20 and the tank 60, then V_(gas) would be equal to V_(tank). If the pressure change in the bladder 20 were 20% as large as that in the tank 60, then V_(gas) would be 5 times greater than V_(tank).

Step 812 derives the value for V_(tbi) from V_(gas), using known values for V_(frame), V_(blad), and V_(bag) and using the calculated value of V_(gas), from step 810.

V _(tbi) =V _(frame) −V _(blad) −V _(bag) −V _(gas)

The valves 102, 106, 104, and 108 can be configured in many ways, including multiple function valves and or manifolds that toggle between distinct states. The depiction herein is made for functional simplicity, not necessarily economy or energy efficiency.

Flow Rate Measurements

FIG. 3 shows the presence of an in-line flow sensor 900 in the output line 612. FIG. 7 shows the external features of an exemplary inline flow sensor 900. Fluid enters an inlet port 901 and exits an outlet port 902, defining a flow path therebetween. An optical sending unit 921 passes light through a proximal housing 932 in general and through optical ribs 933 a and 933 b. A light pattern is read by an optical sensing array 922. A distal body 931 houses an adjustment mechanism 910 that can be turned by the activation of an adjustment gear 909. The proximal housing 932 and the distal body 931 may be secured via one or more fasteners, such as threaded fasteners 934.

The exploded view of FIG. 8 shows the internal parts of the inline sensor 900. A first O-ring 945 and a second O-ring 944 are shown in the illustrated preferred embodiment to create a fluid-tight assembly, although other bonding or sealing methods are also contemplated. A compression spring 941, e.g., a cylindrical or conical helical spring includes a first, fixed end 962 received within an axial bore 963 of the adjustment mechanism 910. A second end 961 of the spring 941 bears against a sensor ball 942, which is received within the flow path. The spring 941 applies a force to the sensor ball 942, urging the ball in the direction opposite to the direction of fluid flow. Alternative elements providing this spring function may be, for example, a resilient band, a resilient or compressible material such as a foam structure, and so forth. The adjustment mechanism 910 may be threaded into an axial opening 964 in the distal body 931, e.g., via external helical threads formed on the distal body 931 which are complimentary and mating with internal helical threads within the opening 964. Alternatively, the spring fixed end 962 may be fixed in position and non-adjustable in such embodiments, the position of the spring fixed end 962 is set by the design in a fixed position of spring pre-load. Rotation of the adjustment mechanism 910 relative to the distal body 931 axially advances or retracts the adjustment mechanism 910, depending on the direction of rotation, and thus causes axial movement of the fixed end 951 of the compression spring 941 to alter the force preload on the sensor ball 942. Alternately, the spring member may be a leaf spring having a first end which is fixed and a second end which is deflectable in the axial or flow direction and which bears against the ball member 942.

The interior of the proximal housing 932 is shown in FIG. 10. The sensor ball 942 (see, e.g., FIG. 9) axially slides within a cavity 955 defined by the assembly. Optional interior ribs 951 and interior flats 952 may be dimensioned in close tolerance to the sensor ball 942 to allow the sensor ball 942 to travel freely within the cavity 955 while remaining centered in the cavity 955. Tapered walls 953 are fabricated with a draft angle, such that the gap around the sensor ball 942 changes as sensor ball 942 is positioned in different positions within cavity 955.

Referring to the section view of FIG. 9, assume that fluid pressure at inlet port 901 is greater than the fluid pressure at outlet port 902, such that fluid flows from higher pressure to lower pressure. Fluid flow will push on the sensor ball 942 against the urging of the spring 941. Depending on the gap between the sensor ball 942 and the proximal housing 932 and depending on the rate of fluid flow, a force will be exerted upon the sensor ball 942. The compression spring 941 is in contact with the sensor ball 942 such that a spring force is generated to the extent that compression spring 941 is compressed. Fluid traverses beyond sensor ball 942 and exits the assembly via outlet port 902. The gap through which fluid flows between the sensor ball 942 and the proximal housing 932 increases as the sensor ball 942 moves towards compression spring 942.

The graph depicted in FIG. 11 shows the forces created by fluid flow and an opposing spring force. As flow rate increases, the force on the sensor ball 942 will increase, which pushes sensor ball 942 in a manner that has two consequences. First, the gap between sensor ball 942 and proximal housing 932 increases, so that the force applied to sensor ball 942 is reduced due to a larger effective area for fluid to travel around the sensor ball 942. Secondly, the sensor ball 942 moves to increase the force applied by the compression spring 941. The sensor ball 942 thus moves until the force of the compression spring 941 is balanced by the force of the fluid flow against the sensor ball 942. An exemplary equilibrium point 851 is shown in FIG. 11, where the force of the compression spring 941 is balanced with the force created by a flow rate of 1 ml per hour and the sensor ball moves to a displacement approximately 0.07 inches (0.18 cm) away from a seated position of sensor ball 942.

In operation, the light source 921, which may be, for example, an LED array, transmits light through the first optical rib 933 a and into the cavity 955. The light incident upon the ball 942 is transmitted through the ball 942 and through the second optical rib 933 b to form a light intensity pattern on the photosensor array 922. The photosensor array 922 may be, for example, a charge-coupled device (CCD) array, photodiode array, complimentary metal oxide semiconductor (CMOS) digital detector array, or the like.

The optical transmitter may include one or more light source elements having a wavelength, for example, in the infrared (IR), visible, or ultraviolet (UV) region and the housing and ball member may be formed of a material that optically transmits light of the light source wavelength. The light source may be an array of light elements, such as LEDs, or laser, etc. The light source may be segmented along the axis or may be a continuous, e.g., scanned or otherwise optically formed beam. The light source may illuminate the detector array along its length simultaneously or by sequentially scanning along its length. The refractive effect of a transparent ball member may have a focusing effect on the light passing therethrough that may be detected by the photosensor array. Alternatively, a nontransmissive ball may be employed and the ball position may be determined by detecting the position of a shadow cast by the ball on the photosensor array. In still further embodiment, the ball member may have reflective surface and the optical sensor array may be positioned to detect light reflected from the surface of said ball.

The output of the photosensor array 922 may be passed via the data line 710 to the processing system 700, which may include a position-detection module or circuitry wherein the axial position of the ball 942 within the channel 955 is determined. The axial position of the ball 922 may in turn be used to determine a flow rate and/or calibrate or correlate ball 922 positions with known flow rates calculated by other means such as plural volume measurements made using the method outlined in FIG. 6 over time, or using the pressure decay method outlined in FIG. 12 and described below.

In certain embodiments, the known flow rates corresponding to axial ball positions may be stored in a memory of the processing system 700, for example, in a table, database, or the like. In such embodiments, when an axial position of the ball 922 is measured, the measured position of the ball may be compared with the table of known flow rates and the flow rate corresponding to the measured axial position is then determined. In other embodiments, calibration measurements of axial ball position and known flow rates may be used to derive an algorithmic formula for mapping a measured axial ball position to a corresponding flow rate. In such other embodiments, when an axial position of the ball 922 is measured, the derived algorithmic formula may then be used to determine the flow rate.

While the present disclosure provides a currently preferred implementation of the system herein, it will be recognized that alternate inline flow sensors are also contemplated.

Flow Rate Calculation

Once the fluid volume has been computed, then multiple measurements made over time will yield knowledge of fluid flow rate, which is, by definition, fluid volume changing over time. Repeated measurements of volume over time provided more and more resolution of average flow rate. The average flow rate and the volume of liquid 40 remaining to be infused can be used to estimate the time at which the fluid volume will be delivered. If the infusion is to be completed within some specified period of time, any error between the specified time and the estimated time can be calculated and the flow rate can be adjusted accordingly.

There are situations where the short-term flow rate is of interest. Rather than make repeated volume measurements over a short period of time, there is an alternative approach. Once the gas volume in bladder 20 is known, then the observation of pressure decay in the bladder can be converted directly to a flow rate. It is important to know that the measurement of pressure decay, by itself, is not adequate to compute flow rate. For example, if the pressure were decaying at a rate of 10% per hour, this information cannot be converted into flow rate, unless the starting gas volume is known. As an example, if V_(gas) has been measured to be 500 ml and the absolute pressure is decaying at a rate of 5% per hour, then the flow rate is 5% of 500 ml per hour or 25 ml per hour. The knowledge of the initial volume is critical to compute fluid flow rate.

The measurement of pressure decay is a simple procedure of observing the time the absolute pressure of P_(bladder) to drop by a small, but significant, amount, preferably for example about 2%. Because the processor 700 is capable of measuring times from microseconds to years, this measurement carries a very wide dynamic range. By observing a 2% drop, the change in pressure is well above the noise floor of the pressure measurement system.

A flow chart outlining an exemplary process 1000 for calculating flow rate by monitoring the rate of pressure decay in the bladder 20 is shown in FIG. 12. At step 1004, the volume of gas in the bladder 20 is calculated as detailed above. At step 1008, the pressure in the bladder 20, P_(bladder) is measured using the sensor 202 at time T1, which is recorded in step 1012. The pressure in the bladder 20 is measured again at step 1016 and the time T2 is recorded at step 1020. The change in pressure, ΔP, between the time T1 and the time T2 is calculated in step 1024 as P_(bladder)−P_(bladder2) and the change in time, ΔT, is calculated as T2-T1 at step 1028. At step 1032, it is determined whether ΔP is greater than some predetermined or prespecified threshold value, e.g., about 2% with respect to P_(bladder1). If ΔP has not reached the threshold value at step 1032, the process returns to step 1016 and continues as described above. If ΔP has reached the threshold value at step 1032, the rate of pressure decay is calculated as ΔP/ΔT at step 1036. The flow rate is then calculated as ΔP/ΔT×V_(gas)−P_(bladder1) at step 1040.

Flow Rate Correlations

The relationship and purpose of having two independent measurement methods for determining flow rate is best described by referring to FIG. 13.

One purpose of the two measurement systems is to calibrate flow measurement 865 with repeated values over time from primary volume measurement 861. Flow measurement 865, e.g., as determined as described above by way of reference to the flow sensor 900, is a measurement of flow rate or the first derivative of fluid quantity with respect to time. If one were to integrate the value of flow measurement 865 over time, the result would be a quantity of fluid. Any errors in this signal would accumulate, providing decreasing volume accuracy over time.

In contrast, an integral signal, such as that from primary volume measurement 861, e.g., calculated using the volume measurement method as described herein, has a fixed error that does not accumulate over time. In fact, as a percentage, the error obtained with an integral signal will decrease over time. As an analogy, if one were to attempt to reach a certain distance in a determined period of time, the use of a speedometer alone would lead to an obvious and significant error. Using this analogy, if one were to use integral measurements, such as those provided by an odometer and a clock, the resultant accuracy would be high.

Flow measurement 865, as described above, operates over a very wide flow rate range and cannot, in any practical way, be calibrated in advance to accommodate manufacturing variances and other environmental factors such as fluid viscosity. For any given fluid flow rate, the signal from flow measurement can be measured and correlated with repeated measurements over time from primary volume measurement 861. For example, if the measurement from flow measurement 865 was observed to a value “x” over a period of ten minutes and a measurement made by primary volume measurement 861 at the beginning of this period was 100 mL and a subsequent measurement made by primary volume measurement 861 at the end of this period was 90 mL, a correlation could be made between flow signal “x” and a flow rate of 10 mL per 10 minutes, or, 60 mL per hour. Flow rate calibration data may be maintained in memory, preferably a nonvolatile memory, of the processing system 700.

Another purpose of the dual measurement system is to distinguish between two sources of fluid directed to the same output. For purposes of distinguishing the source of fluid, assume that flow measurement 865 has been calibrated at various flow rates as described above. If a secondary fluid source 862 is connected to the system, as shown in FIG. 13, and has a fluid driving pressure greater than the fluid within the subsystem for primary volume measurement 861, then the fluid from secondary fluid source 862 will flow towards flow measurement 865 and will block any fluid flow coming from primary volume measurement 861 by the operation of a one way check valve 863. In this case, the signal from primary volume measurement 861 will be unchanging over time. In this circumstance, the non-zero signal from flow measurement 865 will represent fluid flow from the secondary fluid source 862. Alternatively, the flow signal 865 may be integrated to provide an estimate of volume delivered over any period of time. The measurement of volume delivered from secondary fluid source is, in the instance of an intravenous infusion system, an important clinical measurement.

Yet another purpose of the dual measurement system is to detect a condition where gas is expressed from the primary infusion liquid. If a quantity of air leaves the system by way of an in-line air elimination filter 864, then an increased pressure drop will be observed. By itself, this increased pressure drop would indicate that the fluid flow rate increased proportionally. If air were to escape the system from air elimination filter 864, the signal from flow measurement 865 would remain unchanged, providing an indication that the pressure drop should be interpreted as an escape of air, not an increased in fluid flow. In this circumstance, without flow measurement 865, the pressure signal would be interpreted incorrectly.

Yet another purpose of the dual measurement system is to detect a condition where a leak in the pneumatic system exists. If an air leak occurs in the system, a pressure drop will be observed. By itself, this pressure drop would indicate that fluid is flowing from the system. If air were leaking, the signal from flow measurement 865 would be zero, providing an indication that the pressure drop should be interpreted as a leak of air, not as fluid flow. In this circumstance, without flow measurement 865, the pressure signal would be interpreted incorrectly.

The invention has been described with reference to the preferred embodiments. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. 

1. An inline flow sensor, comprising: a conduit defining a flow passageway between a fluid inlet and a fluid outlet; a ball member movable within said flow passageway; a spring for urging said ball member in a direction opposite a direction of fluid flow through said conduit; and an optical sensor for determining a position of said ball member within said conduit.
 2. The inline flow sensor of claim 1, wherein said conduit comprises: a housing defining a cavity having an axis and which is tapered in the direction of said axis; and optionally, means within said cavity for restricting movement of said ball member to movement along said axis.
 3. The inline flow sensor of claim 2, wherein said optical sensor comprises: an optical transmitter positioned on a first side of said housing; and an optical receiver positioned on a second side of said housing opposite said first side.
 4. The inline flow sensor of claim 3, wherein an optical path is created through said ball member.
 5. The inline flow sensor of claim 3, wherein said optical receiver comprises a linear photosensor array extending in a direction parallel to said axis.
 6. The inline flow sensor of claim 5, wherein said photosensor array is selected from the group consisting of a photodiode array, a CCD array, and a CMOS digital detector array.
 7. The inline flow sensor of claim 5, wherein said optical transmitter comprises a linear LED array.
 8. The inline flow sensor of claim 3, further comprising: said optical transmitter comprises a light source having a light source wavelength selected from a wavelength in the IR, visible, or UV region; and said housing and ball member are formed of a material that optically transmits light in said light source wavelength.
 9. The inline flow sensor of claim 1, further comprising: means for adjusting a preload force of said spring on said ball member.
 10. A flow control system for a fluid delivery system of a type for delivering a volume of fluid from a container, the fluid control system comprising: an inline flow sensor fluidically coupled to an output of the fluid delivery system for generating a signal representative of a sensed flow rate of the fluid; means for calculating volume of the liquid remaining in the container; and a computer-based information handling system including: means for monitoring the signal from the inline flow sensor; means for calibrating said inline flow sensor using the signal from the inline flow sensor and successive calculations of volume of the liquid remaining in the container; and means for determining a flow rate of the fluid using one or both of: the successive calculations of volume of the liquid remaining in the container; and the signal from the inline flow sensor.
 11. A method of determining an absolute flow rate of a fluid to be delivered in a fluid delivery system, comprising: sensing a relative flow rate using an inline flow sensor fluidically coupled to an output of the fluid delivery system for generating a signal representative of the sensed relative flow rate of the fluid; and determining the absolute flow rate of the fluid from the sensed relative flow rate of the fluid.
 12. The method of claim 11, wherein determining the absolute flow rate of the fluid comprises: calculating volume of the liquid to be delivered; monitoring the signal from the inline flow sensor; calibrating the inline flow sensor using the signal from the inline flow sensor and one or both of: successive calculations of volume of the liquid to be delivered; and a pressure decay of a pressurized bladder bearing against the liquid to be delivered; and determining the absolute flow rate of the fluid using one or more of: the successive calculations of volume of the liquid to be delivered; the signal from the inline flow sensor; and a pressure decay of a pressurized bladder bearing against the liquid to be delivered.
 13. The method of claim 11, wherein determining the absolute flow rate of the fluid comprises one or more of: comparing the sensed relative flow rate of the fluid to previously stored sensed relative flow rates for known absolute flow rates; and converting the sensed relative flow rate of the fluid to an absolute flow rate using an algorithmic formula mapping sensed relative flow rates to absolute flow rates.
 14. The method of claim 11, wherein the inline flow sensor includes: a conduit defining a flow passageway; a ball member movable within said flow passageway; a spring for urging said ball member in a direction opposite a direction of fluid flow through said conduit; and an optical sensor for determining a position of said ball member within said conduit.
 15. The method of claim 14, wherein determining the absolute flow rate of the fluid comprises comparing known absolute flow rates for ball member positions within said flow passageway.
 16. The method of claim 15, wherein comparing known absolute flow rates for ball member positions within said flow passageway comprises looking up the known absolute flow rates in a look up table.
 17. The method of claim 11, further comprising: independently calculating an absolute flow rate of the fluid as it exits a primary fluid source of the fluid delivery system; and comparing the independently calculated absolute flow rate to the absolute flow rate determined using the inline sensor.
 18. The method of claim 17, further comprising using differences between said independently calculated absolute flow rate of the fluid as it exits the primary fluid source and said absolute flow rate determined using the sensed relative flow rate to identify one or more of: fluid delivered from a secondary fluid source of the fluid delivery system; and a quantity of gas leaving the fluid delivery system.
 19. The method of claim 17, further comprising: placing a flexible bag containing the liquid to be infused within a rigid container of known total volume and containing an inflatable bladder, said inflatable bladder fluidically coupled to a pneumatic system for inflating said bladder; using one or both of volume measurements in said bladder over time and pressure measurements of said bladder over time to calculate said independently calculated absolute flow rate.
 20. The method of claim 19, further comprising using differences between said independently calculated absolute flow rate of the fluid as it exits the primary fluid source and said absolute flow rate determined using the sensed relative flow rate to identify one or more of: fluid delivered from a secondary fluid source of the fluid delivery system; a quantity of gas leaving the fluid delivery system; and a leak in the pneumatic system.
 21. The method of claim 19, wherein said independently calculated absolute flow rate is calculated using a rate of pressure decay of a volume of gas in the inflatable bladder. 